This invention relates to television special effects, and more particularly to an improved architecture for a television special effects generator.
A video image is usually recorded or displayed by sweeping an electron beam horizontally across the face of a camera tube or a picture tube a number of times, from top to bottom, before a vertical retrace is accomplished and the procedure is repeated. The pattern of lines on the face of a camera or picture tube is called a "raster". If the image is created by horizontal lines that are immediately adjacent each other and incrementing downward monotonically, it is known as progressive raster scanning or non-interlaced video.
However, to reduce the bandwidth required for the transmission of the image data while preserving as much resolution as possible, commercial television standards make use of a technique known as "interlace scanning". In this approach, a complete frame of video with full resolution is made of two consecutive fields, each with half as many lines, such that the lines of one field fall between those of the other field. One of these two fields contains, in effect, all of the odd lines of a normal frame of video, while the other contains all of the even lines. The two interlaced fields create an image with an effective sampling rate that is twice that of each field separately, and therefore for a given bandwidth the vertical resolution is doubled relative to each of them separately.
Effective as they are at conserving bandwidth while preserving resolution, interlaced television video signals present a number of problems in modern digital television special effects equipment. A variety of video special effects can cause apparent motion that creates a problem in connection with interlaced video. These include synthetic light reflections, adding borders, defocus effects, key glow generation and mosaic tiling. In each of these operations, apparent motion is introduced into the video image and this apparent motion shows up as undesirable artifacts if the interlaced fields are operated on independently of each other.
Higher quality video special effects systems attempt to use the information from two or more fields to create output images and thus produce images that utilize the full resolution capability of the interlace standard. To avoid distortions produced by motion or apparent motion, these systems adaptively switch between full frame and single field operation, depending on whether motion is detected locally on a pixel by pixel basis.
U.S. Pat. No. 4,472,732 to Bennett et al for a "System for Spatially Transforming Images", hereby incorporated by reference, discloses a video special effects system that includes a deinterlace filter (1332, FIGS. 13 and 18). This filter includes motion detection on a pixel-by-pixel basis that is used to determine the content of an output frame. Motion is detected by comparing a pixel's Y, I and Q values in a most recent field with the values two fields earlier. If these values differ by more than a threshold amount, a motion signal is generated. In the absence of this motion signal, in generating a deinterlaced frame output, the same pixel on the next line will have its value determined by the next most recent field of interlaced video. However, in the presence of this motion signal, the value of the same pixel on the next line is based on the average of the pixels above and below it in the most recent field. Thus, this deinterlace filter produces a progressive scan frame of video in a manner that adaptively filters out field-to-field motion.
In the architecture described in U.S. Pat. No. 4,472,732, the deinterlaced output of the deinterlace filter is filtered and then used immediately in an interpolation process that produces an interlaced output again.